What helped the Republicans more than redistricting was the tendency of Democratic voters to be clustered in black, Hispanic and "gentry liberal" neighborhoods in major metropolitan areas. This clustering has produced huge majorities that have made many large and medium-size states safely Democratic at the presidential level. Barack Obama won 56% or more in 13 states and the District of Columbia with 179 electoral votes, leaving him only 91 votes short of a majority. Mitt Romney, in contrast, won 56% or more in states with only 125 electoral votes.
But clustering works against Democrats in the House. According to figures compiled by Polidata Inc. for National Journal and "The Almanac of American Politics" (of which I am a co-author), Mr. Obama won 80% or more of the vote in 27 congressional districts and between 70% and 79% in 34 more. Mr. Romney won 80% in only one district and between 70% and 79% in 18 more. That left enough Republican votes spread around in the other 355 districts to enable Mr. Romney to carry 226 congressional districts to Obama's 209.
All of the Democrats' House popular-vote margin came from the 36 black-dominated and 31 Hispanic-dominated districts. Democrats carried the popular vote in black-dominated districts 80%-17% in 2012. They made significant gains in Hispanic-dominated districts, which George W. Bush lost by 11% but Mitt Romney lost by 32%. Mr. Bush's "Family values don't stop at the Rio Grande" is a more attractive message than Mr. Romney's "self-deportation."
Still, the House popular vote in the large majority of districts—368 in 2012, 369 in 2004—not dominated by blacks or Hispanics was almost the same in those two years. Republican candidates carried such districts 53%-44% in 2004 and 52%-46% in 2012.